This application claims the benefit of Korean Application No. 00-25329, filed May 12, 2000, in the Korean Patent Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of manufacturing a lithium secondary cell, and more particularly, to a method of manufacturing a lithium secondary cell without a process of extracting a plasticizer by using an organic solvent.
2. Description of the Related Art
In general, a non-aqueous lithium secondary cell includes an anode, an electrolyte formed from at least one lithium salt dissolved in an organic solvent and a cathode of an electrochemical active material which is a transition metal chalcogenide. During discharging, lithium ions from the anode generate electrical energy and move to an electrochemical active material of the cathode that absorbs the lithium ions through the liquid electrolyte. During charging, the ion flow is reversed so that the lithium ions move from the electrochemical active material to the anode through the electrolyte to then be plated. Such non-aqueous lithium secondary cells are disclosed in U.S. Pat. Nos. 4,472,487, 4,668,595, 5,028,500, 5,441,830, 5,460,904 and 5,540,741.
To solve the problems of dendrite and sponge lithium growth, a metallic lithium anode is replaced with a carbon anode, e.g., coke or graphite, to which lithium ions are intercalated to form LixC6. During charging, lithium ions come from a cathode active material and intercalate into the carbon anode to form a lithium-carbon intercalation compound. During the operation of such a cell, like in the cell having a metallic lithium anode, lithium ions move from the carbon anode to the cathode that absorbs the lithium ions through an electrolyte. During recharging, the lithium ions return to the anode to then be intercalated into the carbon. Since metallic lithium is not present in the cell, the anode does not melt at all even under severe conditions. Also, since lithium is recombined into the anode by intercalation rather than by plating, dendrite and sponge lithium growth does not occur.
Recently, lithium secondary cells using a porous polymeric matrix as a separator have emerged, and it has been proven that conductivity could improve by using the porous polymeric matrix. One of the methods for preparing a porous polymeric matrix includes the steps of forming a polymer structure containing a plasticizer such as dibutyl phthalate and forming voids in the polymer by removing the plasticizer. Further, the method includes adding plasticizer to the electrode active material composition for the purpose of forming pores in the electrode plates, to increase the amount of electrolytic solution impregnated in the cell, and enhancing the processing characteristics when the electrode plates and the separator are laminated together. 50 wt % or less of the plasticizer may be contained in the lithium secondary cell before it is removed. Currently, methods of removing the solvent include extraction using another organic solvent such as dimethyl ether, methanol or cyclohexane. In general, in the fabrication of a lithium secondary cell, an electrolytic solution containing an electrolytic solvent and salt is added for the purpose of activating a lithium secondary cell precursor after removing the plasticizer.
As described above, the lithium secondary cell using a plasticizer has an excellent electrochemically operating capacity. However, since the solvent used in extracting the plasticizer is a harmful organic solvent, it may cause environmental pollution. Also, performing extraction makes a manufacturing time longer and lowers a manufacture yield, which increases the manufacturing cost of a lithium secondary cell. Further, the loading amount of the electrode active material on the electrode plate is decreased because of the plasticizer.
To solve the above problems, it is an object of the present invention to provide a method of manufacturing a lithium secondary cell without a step of extracting a plasticizer using an organic solvent.
Accordingly, to achieve the above and other objects, there is provided a method of manufacturing a lithium secondary cell comprising preparing an anode precursor and a cathode precursor by coating collectors with electrode compositions, each not containing a plasticizer, preparing a separator precursor by coating both sides of a porous polymer film which is not gelled by an electrolytic solution with slurry containing an ion conductive polymer and the plasticizer, laminating the cathode and anode precursors and the separator precursor to prepare a cell precursor, and activating the cell precursor by injecting the electrolytic solution into the cell precursor.
The cell precursor is preferably formed in a bicell structure in which the cathode precursor, the separator precursor, the anode precursor, the separator precursor and the cathode precursor are stacked in sequence.
In the method of manufacturing a lithium secondary cell, each of the electrode compositions may further include an alcoholic solvent selected from the group consisting of methanol, ethanol, isopropanol and mixtures thereof.
The porous polymer film which is not gelled by an electrolytic solution is preferably a porous polyethylene film or a porous polyethylene film having a porous polypropylene film stacked on both surfaces thereof.
The plasticizer is preferably at least one selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethoxyethane, dibutyl phthalate, dimethoxyethane, diethyl carbonate, dimethoxyethane, dipropyl carbonate and mixtures thereof.
The copolymer of vinylidenefluoride and hexafluropropylene is preferably coated to a thickness of 1 to 50 xcexcm.